Summary: Background: Human brain continues to develop during the first few years of postnatal life. Since the developing brain is uniquely sensitive to damage following virus infection, administration of neurovirulent vaccines to infants can place the child's nervous system at increased risk for vaccine related injury. There is a need to develop a test to assess the vaccine's human neurotoxicity potential, in order to develop the safest vaccines possible. Such an effective test currently does not exist for most new vaccines. Vaccine neurotoxicity concerns have been raised for mumps virus, vaccinia virus (Smallpox vaccine), influenza virus, human parainfluenza virus III, poliovirus, dengue virus, Japanese encephalitis virus and human immunodeficiency virus. Thus, neurotoxicity safety tests (NVST) are needed to identify a multitude of potentially neurovirulent vaccines that may cause CNS disease following vaccination. Progress: 1. Mumps vaccine: A newborn rat neurotoxicity test was developed we are proceeding with international validation studies with NIBSC in England and with health authorities in Brazil. We have also discovered virus genomic changes associated with neurotoxicity alterations, as well as molecular markers of host neuronal damage following infection. Two manuscripts summarizing these results are in preparation. 2. Influenza vaccine: Although rare, CNS events associated with wild type influenza virus can occur. In this project we are developing an assay to evaluate the relative neurotoxicity of influenza viruses (wild type and vaccine) using our newborn rat model. Our data show that we can differentiate between wild type influenza virus and vaccine strains in our neurotoxicity test. These results have been submitted for review. 3. Smallpox vaccines: We have developed a newborn mouse model for differentiating the toxicity of vaccinia virus strains. We can differentiate between vaccinia strains of greater or lesser neurotoxicity, suggesting this test will have utility as a pre-clinical toxicity test for new smallpox vaccines. A manuscript summarizing these results is in preparation. 4. We have begun to develop a mouse model for measles virus neurotoxicity testing using transgenic mice.